由二根對稱排列的(de)導線組成通信回(hui)路(lu)(lu)。分(fen)高頻(pin)和低頻(pin)兩種。前者(zhe)高傳輸(shu)頻(pin)率可(ke)達 800千赫，相應為(wei)在一(yi)(yi)個(ge)(ge)回(hui)路(lu)(lu)中可(ke)開(kai)通180路(lu)(lu)電(dian)話；后者(zhe)高傳輸(shu)頻(pin)率一(yi)(yi)般小(xiao)于(yu)252千赫，相當(dang)于(yu)一(yi)(yi)個(ge)(ge)回(hui)路(lu)(lu)中可(ke)開(kai)通60路(lu)(lu)電(dian)話。　對稱通信電(dian)纜的(de)電(dian)磁場呈開(kai)放狀態（圖1），在高頻(pin)下(xia)回(hui)路(lu)(lu)的(de)衰(shuai)減(jian)和損耗較大(da)，回(hui)路(lu)(lu)間相互干擾和外界干擾都(dou)較大(da)，難于(yu)提高傳輸(shu)頻(pin)率和容量。

　　長(chang)途對(dui)稱通(tong)信(xin)電(dian)纜由不同(tong)數量和不同(tong)絕(jue)(jue)緣結構(gou)的(de)(de)四線組(zu)構(gou)成。四線組(zu)的(de)(de)常用(yong)(yong)形(xing)式(shi)為星絞(jiao)組(zu)，也有(you)的(de)(de)采用(yong)(yong)復對(dui)絞(jiao)形(xing)式(shi)（圖2）。絕(jue)(jue)緣有(you)紙帶(dai)絕(jue)(jue)緣、紙-繩(sheng)(sheng)（紙帶(dai)和紙繩(sheng)(sheng)）絕(jue)(jue)緣、聚(ju)乙烯(xi)繩(sheng)(sheng)-帶(dai)絕(jue)(jue)緣、聚(ju)苯乙烯(xi)繩(sheng)(sheng)－帶(dai)絕(jue)(jue)緣和泡沫聚(ju)乙烯(xi)絕(jue)(jue)緣等多(duo)種。高(gao)頻長(chang)途對(dui)稱通(tong)信(xin)電(dian)纜傳輸頻率高(gao)，所以(yi)(yi)對(dui)電(dian)纜的(de)(de)結構(gou)性能要(yao)求(qiu)較高(gao)。一(yi)般采用(yong)(yong)繩(sheng)(sheng)－帶(dai)絕(jue)(jue)緣的(de)(de)星絞(jiao)四線組(zu)結構(gou)。絕(jue)(jue)緣材料常用(yong)(yong)聚(ju)苯乙烯(xi)、聚(ju)乙烯(xi)。紙帶(dai)紙繩(sheng)(sheng)絕(jue)(jue)緣一(yi)般用(yong)(yong)于252千赫(he)以(yi)(yi)下(xia)的(de)(de)低頻對(dui)稱通(tong)信(xin)電(dian)纜。

　　電(dian)(dian)(dian)(dian)(dian)(dian)話(hua)(hua)(hua)電(dian)(dian)(dian)(dian)(dian)(dian)纜(lan)(lan)是用于市內、近(jin)郊和(he)(he)工礦(kuang)企(qi)業等較(jiao)小范圍的(de)(de)(de)區域電(dian)(dian)(dian)(dian)(dian)(dian)話(hua)(hua)(hua)連接(jie)的(de)(de)(de)對(dui)(dui)稱通(tong)信電(dian)(dian)(dian)(dian)(dian)(dian)纜(lan)(lan)，常(chang)稱市內電(dian)(dian)(dian)(dian)(dian)(dian)話(hua)(hua)(hua)電(dian)(dian)(dian)(dian)(dian)(dian)纜(lan)(lan)。其(qi)(qi)主要(yao)特點為(wei)(wei)(wei)對(dui)(dui)數多（多可達數千對(dui)(dui)，一(yi)(yi)(yi)般(ban)(ban)為(wei)(wei)(wei)數百或(huo)數十(shi)對(dui)(dui)）。由(you)于使(shi)用頻率低，通(tong)信距離近(jin)，因(yin)此線(xian)(xian)(xian)徑(jing)較(jiao)長途(tu)通(tong)信電(dian)(dian)(dian)(dian)(dian)(dian)纜(lan)(lan)細，一(yi)(yi)(yi)般(ban)(ban)為(wei)(wei)(wei)0.5毫米。電(dian)(dian)(dian)(dian)(dian)(dian)話(hua)(hua)(hua)電(dian)(dian)(dian)(dian)(dian)(dian)纜(lan)(lan)的(de)(de)(de)線(xian)(xian)(xian)組結構(gou)(gou)有對(dui)(dui)絞(jiao)組、星絞(jiao)組和(he)(he)復對(dui)(dui)絞(jiao)組3種。按其(qi)(qi)線(xian)(xian)(xian)心(xin)(xin)絕(jue)緣和(he)(he)護層(ceng)(ceng)材料(liao)可分(fen)(fen)(fen)為(wei)(wei)(wei)紙絕(jue)緣鉛護套(tao)電(dian)(dian)(dian)(dian)(dian)(dian)纜(lan)(lan)、聚乙烯(xi)絕(jue)緣組合(he)護層(ceng)(ceng)電(dian)(dian)(dian)(dian)(dian)(dian)纜(lan)(lan)、油膏充填防水(shui)電(dian)(dian)(dian)(dian)(dian)(dian)纜(lan)(lan)、全塑電(dian)(dian)(dian)(dian)(dian)(dian)纜(lan)(lan)等。　電(dian)(dian)(dian)(dian)(dian)(dian)話(hua)(hua)(hua)電(dian)(dian)(dian)(dian)(dian)(dian)纜(lan)(lan)中二根絕(jue)緣導(dao)線(xian)(xian)(xian)心(xin)(xin)按一(yi)(yi)(yi)定(ding)節距絞(jiao)合(he)成(cheng)(cheng)對(dui)(dui)構(gou)(gou)成(cheng)(cheng)一(yi)(yi)(yi)個(ge)絕(jue)緣線(xian)(xian)(xian)對(dui)(dui)。線(xian)(xian)(xian)對(dui)(dui)中二根絕(jue)緣導(dao)線(xian)(xian)(xian)心(xin)(xin)的(de)(de)(de)顏(yan)色(se)(se)不同(tong)（一(yi)(yi)(yi)般(ban)(ban)為(wei)(wei)(wei)紅色(se)(se)和(he)(he)白(bai)色(se)(se)），以便(bian)接(jie)線(xian)(xian)(xian)時(shi)區分(fen)(fen)(fen)。電(dian)(dian)(dian)(dian)(dian)(dian)話(hua)(hua)(hua)電(dian)(dian)(dian)(dian)(dian)(dian)纜(lan)(lan)的(de)(de)(de)纜(lan)(lan)心(xin)(xin)結構(gou)(gou)一(yi)(yi)(yi)般(ban)(ban)分(fen)(fen)(fen)為(wei)(wei)(wei)同(tong)心(xin)(xin)式和(he)(he)單(dan)位式兩種（圖3）。同(tong)一(yi)(yi)(yi)層(ceng)(ceng)中相鄰線(xian)(xian)(xian)對(dui)(dui)的(de)(de)(de)絞(jiao)合(he)節距應不相同(tong)，以減小通(tong)話(hua)(hua)(hua)時(shi)的(de)(de)(de)相互影響(xiang)。在每一(yi)(yi)(yi)層(ceng)(ceng)中都(dou)設有一(yi)(yi)(yi)個(ge)標志對(dui)(dui)（分(fen)(fen)(fen)別為(wei)(wei)(wei)藍色(se)(se)和(he)(he)白(bai)色(se)(se)），便(bian)于接(jie)線(xian)(xian)(xian)時(shi)辨(bian)認(ren)。在80對(dui)(dui)以上的(de)(de)(de)電(dian)(dian)(dian)(dian)(dian)(dian)纜(lan)(lan)中往往置有預(yu)備(bei)線(xian)(xian)(xian)對(dui)(dui)以替(ti)換(huan)不合(he)格(ge)線(xian)(xian)(xian)對(dui)(dui)。單(dan)位式電(dian)(dian)(dian)(dian)(dian)(dian)纜(lan)(lan)以50對(dui)(dui)或(huo)100對(dui)(dui)及相應的(de)(de)(de)預(yu)備(bei)對(dui)(dui)絞(jiao)合(he)成(cheng)(cheng)一(yi)(yi)(yi)個(ge)基本單(dan)位，再由(you)若干個(ge)基本單(dan)位絞(jiao)合(he)成(cheng)(cheng)電(dian)(dian)(dian)(dian)(dian)(dian)纜(lan)(lan)心(xin)(xin)。經(jing)干燥后擠(ji)壓上護套(tao)制成(cheng)(cheng)電(dian)(dian)(dian)(dian)(dian)(dian)纜(lan)(lan)（見圖）。

　　架空敷設的電(dian)話電(dian)纜需用自承(cheng)(cheng)式，由鋼索承(cheng)(cheng)受整個電(dian)纜的重力。20世(shi)紀60年代(dai)以來，研制出(chu)在塑料電(dian)纜中填充油膏以防(fang)止潮氣(qi)和(he)水(shui)分的防(fang)水(shui)型電(dian)纜。

　　為(wei)了提高(gao)通(tong)信電(dian)纜的防潮性(xing)、穩定性(xing)，20世紀(ji)70年代以來，在市內電(dian)話電(dian)纜中廣泛采用綜合型電(dian)纜護層(ceng)(ceng)。其基本類型有3種(zhong)。①Alpeth：纜心外擠(ji)壓一(yi)(yi)層(ceng)(ceng)聚(ju)乙(yi)(yi)烯(xi)護層(ceng)(ceng)，再(zai)搭接地縱向包裹0.2毫米皺(zhou)紋(wen)鋁帶，充以聚(ju)異丁烯(xi)絕緣復合物后外敷(fu)一(yi)(yi)層(ceng)(ceng)熱(re)塑(su)性(xing)膠(jiao)粘層(ceng)(ceng)，外面擠(ji)壓一(yi)(yi)層(ceng)(ceng)聚(ju)乙(yi)(yi)烯(xi)護套(tao)(tao)。②Stalpeth：皺(zhou)紋(wen)鋁帶外縱包一(yi)(yi)層(ceng)(ceng)皺(zhou)紋(wen)鋼帶，外覆一(yi)(yi)層(ceng)(ceng)熱(re)塑(su)性(xing)材料(liao)(liao)，外層(ceng)(ceng)再(zai)擠(ji)壓一(yi)(yi)層(ceng)(ceng)聚(ju)乙(yi)(yi)烯(xi)護套(tao)(tao)。③Lepeth：纜心上包覆一(yi)(yi)層(ceng)(ceng)聚(ju)乙(yi)(yi)烯(xi)護套(tao)(tao)，其外敷(fu)一(yi)(yi)層(ceng)(ceng)粘性(xing)熱(re)塑(su)性(xing)材料(liao)(liao)，外面擠(ji)壓一(yi)(yi)層(ceng)(ceng)鉛管。